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Questions and Answers

What type of reservoirs hold more oil?

Siliciclastic reservoirs hold more oil.

Any grain not classified as either quartz or feldspar is called a lithoclast.

True

What does Dott classification refer to?

The Dott classification refers to the use of a ternary diagram and percentage of matrix to classify sandstone and mudstone types.

What are the two main processes of diagenesis that produce the rock known as 'lithification'?

<p>The two main processes that lead to lithification are compaction and cementation.</p> Signup and view all the answers

What is the main cause of overpressure in sedimentary rocks?

<p>Differential compaction of permeable sands and impermeable clays and silts is the primary cause of overpressure in sedimentary rocks.</p> Signup and view all the answers

What type of quartz is often found in the first mixing stage of silica precipitation?

<p>The first mixing stage often results in the precipitation of silica as Opal-CT, which is a form of amorphous silica.</p> Signup and view all the answers

What does the term 'syntaxial' refer to in relation to quartz overgrowth cement?

<p>Syntaxial quartz overgrowth cement signifies that the quartz cement grows in the same crystal orientation as the original detrital quartz grain.</p> Signup and view all the answers

Syntaxial quartz growth only occurs in lithic sandstones.

<p>False</p> Signup and view all the answers

How can the rate of burial impact porosity in sandstones?

<p>The rate of burial influences the amount of porosity in sandstones. Faster burial rates can lead to significant porosity development, whereas slower burial enables more extensive cementation, resulting in lower porosity.</p> Signup and view all the answers

What are the two primary feldspars and their chemical formulas?

<p>The two primary feldspars are K-feldspar, with the formula KAlSi₃O₈, and plagioclase, which is a mixture of albite (NaAlSi₃O₈) and anorthite (CaAl₂Si₂O₈).</p> Signup and view all the answers

What is the main source of silica cement in rapidly subsiding basins?

<p>In rapidly subsiding basins, the main source of silica cement is often cooling formation waters.</p> Signup and view all the answers

What is the relationship between temperature and silica solubility?

<p>Silica solubility decreases as temperature decreases.</p> Signup and view all the answers

What are the two main types of clay minerals often associated with coatings on clastic grains in sandstones?

<p>The two main types of clay minerals often coating clastic grains in sandstones are illite and chlorite.</p> Signup and view all the answers

How does the presence of glauconite affect the porosity and permeability of sandstones?

<p>Glauconite generally reduces the porosity and permeability of sandstones.</p> Signup and view all the answers

What are the common names for glauconite-impregnated sandstones and what are the conditions under which they form?

<p>These sandstones are often called 'greensands' and are formed in marine environments where there is a low rate of sediment deposition, often under conditions of relative quiet water and oxygen-depleted conditions. They are also associated with phosphorite deposits.</p> Signup and view all the answers

Glauconite is universally found in both marine and non-marine sedimentary rocks.

<p>False</p> Signup and view all the answers

Weathering can transform solid rock into clay minerals but not transform clay minerals into solid rock.

<p>True</p> Signup and view all the answers

What is the general term for the breakdown and alteration of rocks and minerals at or near the Earth's surface?

<p>The general term for the breakdown and alteration of rocks and minerals is weathering.</p> Signup and view all the answers

What type of organic matter is thought to be involved in the formation of glauconite?

<p>Microbial decomposition of organic matter is thought to be key to the formation of glauconite minerals.</p> Signup and view all the answers

The process of glauconite formation typically begins with iron-rich smectite.

<p>True</p> Signup and view all the answers

What type of clay dominates in deep marine fan environments?

<p>Smectite clay is the dominant type of clay found in deep marine fan environments.</p> Signup and view all the answers

Illite is the most stable clay mineral in deep burial environments.

<p>True</p> Signup and view all the answers

How does a change in clay mineral composition impact overpressure formation?

<p>The smectite-to-illite transition in shales and mudstones can create overpressure.</p> Signup and view all the answers

Smectite-to-illite transition can also lead to increased permeability.

<p>False</p> Signup and view all the answers

What is the major effect of clay diagenesis on shale/mudstone systems?

<p>Clay diagenesis in shale/mudstone systems significantly reduces permeability.</p> Signup and view all the answers

What is the key factor that influences the type of clay mineral formed during deep burial and diagenesis?

<p>The initial composition of the clay minerals present, particularly the ratio of K-feldspar to kaolinite, plays a major role in determining the type of clay mineral formed during deep burial.</p> Signup and view all the answers

What is the importance of understanding the rate of burial in relation to porosity and permeability in reservoir rocks?

<p>The rate of burial significantly affects porosity and permeability in reservoir rocks, often influencing the quality of a reservoir.</p> Signup and view all the answers

What is the major source of organic matter in sedimentary rocks?

<p>Kerogen is the major source of organic matter in sedimentary rocks.</p> Signup and view all the answers

What are the two main processes involved in the maturation of organic matter?

<p>Catagenesis and metagenesis are the two main processes involved in organic matter maturation.</p> Signup and view all the answers

What is the key mineral that becomes more uniformly crystalline during maturation?

<p>Illite is known to become more uniformly crystalline during the maturation process.</p> Signup and view all the answers

What is the name of the organic material that can form proto-graphite during maturation?

<p>The organic material that can form proto-graphite during maturation is known as 'bitumen', although it can be difficult to distinguish from other organic matter.</p> Signup and view all the answers

Overmature shales are often characterized by low electrical conductivity.

<p>False</p> Signup and view all the answers

What are the two main factors that influence the diagenesis of sedimentary rocks?

<p>The two main factors are the mineral assemblage and the depositional environment.</p> Signup and view all the answers

Name two examples of the impact of diagenesis on reservoir quality.

<p>Two examples of the impact of diagenesis are compaction, which can reduce porosity, and cementation, which can also decrease porosity and block pore throats.</p> Signup and view all the answers

The timing of cementation and compaction can significantly affect the final formation of a reservoir rock.

<p>True</p> Signup and view all the answers

What is the key to understanding how diagenetic processes influence the formation of different reservoir types?

<p>Understanding the timing of specific diagenetic events is crucial for understanding how reservoir properties are affected.</p> Signup and view all the answers

What is one of the main differences between a 'greensand' and a regular sandstone?

<p>Greensands are sandstones that are predominantly impregnated with glauconite minerals, giving them a characteristic green color.</p> Signup and view all the answers

What type of sedimentary environment are greensands typically found in?

<p>Greensands are typically associated with marine environments, particularly shallow continental shelf settings.</p> Signup and view all the answers

What is the main reason why glauconite can significantly reduce the permeability of reservoir rocks?

<p>Glauconite, being a relatively soft mineral, is easily compacted during burial, which can decrease the size of pore throats and hinder fluid flow.</p> Signup and view all the answers

Study Notes

Diagenesis of Siliciclastics

  • This is a course on sedimentary rocks, specifically the diagenesis of siliciclastics.
  • The course is taught by Andy Chater PhD, P.Geo.
  • The course dates are November 30th and December X, 2024.
  • The course is GEM 3114, at the University of Guyana.

Carbonate Oil Reservoirs

  • Carbonate oil reservoirs are widespread in the world's largest petroleum-producing areas.
  • This is based on a 2005 compilation (of data).

Siliciclastic Reservoirs

  • Siliciclastic reservoirs hold significantly more oil than carbonate reservoirs.
  • This is based on a 2005 compilation (of data).

Sandstone (Arenite) Composition

  • Sandstones (arenites) are categorized based on composition using a ternary diagram.
  • Lithic grains are any grains not quartz or feldspar.
  • Types include quartz arenite (>90% quartz), arkosic arenite (>10% feldspar), and lithic arenite (>10% rock fragments).
  • Sandstones with these compositions are rare.

Wackes

  • The gradation between sandstones and mudstones is clearly defined as wackes.
  • Wackes are characteristic of greywackes.
  • A classification system for wackes is the Dott Classification.

Sandstone Diagenetic Reactions

  • Sandstone diagenesis is influenced by burial temperature and organic solvents (which affect pH and alkalinity).
  • Reactions depend on the temperature window, starting from early diagenetic reactions at 20°C.
  • Diagenetic reactions have different physical effects like compaction and bioturbation.
  • There are different carbonate system types.
  • Diagenesis occurs at different stages of burial into the top of the oil window.
  • This diagenesis occurs at high pH and low PCO2.
  • Deep burial diagenesis has internal buffering by carbonate equilibria with high PCo₂.

Compaction and Cementation

  • Compaction and cementation are both processes of diagenetic lithification.
  • Compaction involves a reduction in water content from 50-60% to 10-20% during lithification.
  • Cementation is a different process from compaction.

Differential Compaction

  • Differential compaction of permeable sands and impermeable clays and silts is a significant process for overpressures and clay changes in the rock.
  • Gravels, sands, silts, and clays compact and are cemented into different rock types.

Sandstone Grains

  • Representatives of common grains and deformed grains are found in sandstones.
  • Common grains include deformed and smeared ductile grains, advanced brittle fracture of micas, and pressure solution along quartz-quartz contacts.

Sandstone Composition

  • Most sandstones are quartz arenites or arkosic quartz arenites with few lithic clasts.

Burial Compaction

  • Simple burial compaction leads to selective grain deformation, more grain contact, brittle fracturing, and porosity loss.

Chemical Compaction

  • Chemical compaction of fine sand involves clay-rich rock fragments, smeared quartz cement, and the development of stylolytic structures.

Silica Cement

  • Subsurface mixing of saline and meteoric waters can precipitate silica cements - this happens in a mixing zone.
  • The occurrence and location of the mixing zone can change over time.
  • Seawaters have higher silica content than freshwater.
  • Mixing produces opal then syntaxial quartz.
  • Opal-CT is amorphous cristobalite and tridymite that grows with chalcedony in chert.
  • Microbial opal-ct is found as coatings on very small spheroids.

Opal-CT Differentiation

  • Opal-CT can only be differentiated from opal-A (siliceous ooze) by XRD.

Quartz Grain Coating

  • An example of "micron" silica as a quartz grain coating is either opal-CT, cryptocrystalline quartz or microquartz.

Illite-Smectite Coatings

  • Besides opal-CT, many detrital quartz grains, especially smaller grains, have thin coatings of illite-smectite stained with iron oxides and hydroxides.
  • Coating can inhibit syntaxial quartz overgrowth to some degree.

Syntaxial Quartz Overgrowth

  • Syntaxial quartz overgrowth cement means that the quartz grew continuously with the detrital grain's orientation.
  • Initially, quartz cements elongate grains but evolve into the shape of a quartz crystal.

Lithic Sandstone

  • Syntaxial quartz overgrowth is prevalent in lithic sandstones.
  • Brown rims of clay and iron oxides are also present in these samples.
  • There are remnants of porosity.

Sandstone With Overgrowths and Cement

  • This sandstone shows abundant evidence of syntactic quartz overgrowths from original grain boundaries.
  • Later calcite cement is also observed in the image.

Quartzite Formation

  • Syntaxial quartz overgrowth cement can reduce porosity to near zero, creating quartzite.
  • This is visually apparent in the image as black quartz in extinction.

Euhedral Quartz

  • Euhedral quartz grains mimic a quartz crystal with syntaxial overgrowths.

Kaolinite and Pyrite

  • SEM (scanning electron microscopy) image shows euhedral quartz growths with pyramidal terminations that are associated with kaolinite (small books) and pyrite (spherical framboids).

Eroded Quartz Grains

  • Sometimes, quartz grains are eroded from a previous sandstone that has been uplifted and weathered.

Silica Cement Sources

  • Silica cement in sandstones depends on quartz framework grain space and the silica supply window during different tectonic events.
  • Rift basins and collision basins have different cementation characteristics and little silica cement.
  • Intracratonic basins, foreland basins, and passive-margin basins have most quartz cement.
  • Burial depths and formation water from cooling of regional water systems influence silica cement precipitation.

Silica Solubility

  • Silica is less soluble as the temperature decreases.

Stress on Grain Boundaries

  • Stress on grain boundaries makes silica a cement in sandstones.

Feldspar Diagenesis

  • Silica also comes from the destructive diagenesis of feldspars in arkosic or lithics.
  • Plagioclase feldspars (albite and anorthite) and K-feldspars can be hydrolized to clay minerals.
  • The hydrolysis reactions generate silica in solution.

Clay-Filled Pits

  • Clay-filled pits are almost certainly K-feldspars before diagenesis.
  • Examples of this include the Roraima Sandstone, Guyana sample.

Formation Fluid Flow and Clay

  • Clay has been washed away in the Roraima sample by formation fluid flow.

Common Feldspar Composition

  • Common feldspars, like K-feldspars, have varied stability depending on composition.
  • These stabilities control the diagenesis and transformation into clay minerals like illite.

Unaltered Microcline K-Feldspar

  • Unaltered microcline feldspar (cross-hatch texture) is observed with quartz (grey colors).

Unaltered Plagioclase

  • Unaltered plagioclase is observed in thin section.
  • It shows characteristic styles of twinning.

Illite Formation

  • Illite formation from K-felspar occurs during diagenesis.
  • SEM images show how small the newly formed illite crystals are.

Albitized K-Feldspar

  • In certain conditions, some K-feldspar is albitized, where Na atoms replace some K atoms.
  • This is evident in the pore structure from the center grain.
  • Other parts of the K-feldspar grain is present in the sample but with dissolved porosity.

Carbon Dioxide and Feldspar

  • Carbon dioxide from maturing black shales accelerates feldspar dissolution, and this happens before oil generation.

SiO2 and Ca-Rich Cements

  • Carbon dioxide influx also propels the formation of silica (SiO2) and calcium-rich cements.
  • With a magnesium source, these cements can be dolomite cements.

North Sea Model

  • A model from the Magnus Oil Field in the North Sea illustrates pH reduction from CO2 influx and the buffering by K-feldspar dissolution.

Silica, Temperature and Changes in K-Feldspar Rich Sand

  • Temperature and meteoric water have a measurable impact the formation and stability of K-feldspar in sand.

Smectite to Illite Transition

  • The smectite to illite transition happens at the onset of oil generation temperature.
  • Smectite-rich samples tend to transition faster and at shallower depths.

Clay Mineral Diagenesis in Shales

  • Diagenetic clay precipitation significantly reduces permeability.
  • The main control is temperature.

Shale Diagenesis

  • Compaction, clay water movement and re-alignment, diagenetic transformation of clays, illitization, overgrowth and cement precipitation are main aspects of shale diagenesis.
  • The presence of these processes influence the macroscopic properties of shale, like microstructural aspects like increase in clay crystallinity, preferred orientation, increased strength and stiffness, and porosity reduction/development.

Organic Matter Diagenesis

  • Organic matter diagenesis involves the removal of functional groups, catagenesis (oil gen), oil migration and cracking into methane (CH4), and aromatization and structural reorganization of the C skeleton.
  • Loss of fluorescence, increase in reflectance, and variation in spectroscopic response are some microstructural effects.

Overmature Marcellus Shales

  • These overmature Marcellus shales, with 3-7% TOC and Ro = 4+, exhibit resistivity of less than 0.1 ohm-meter.

Core Mineralogy

  • The core mineralogy in the earlier slide includes quartz, illite, calcite, albite, pyrite, and chlorite.

Bitumen and Proto-Graphite

  • Only bitumen and proto-graphite are left in the overmature Marcellus shales, connecting through minute pore spaces and grain boundaries.
  • This makes the core highly conductive.

Marcellus Shale Impact

  • The high maturity in Marcellus shales led to the formation of proto-graphite.

Canning Basin

  • Similar compaction and burial history in Canning Basin shales exhibit different fabric and timing of diagenesis, as early diagenesis prevents compaction.
  • Fabric development, strength, and velocity anisotropy are impacted.
  • This area also has some influence from mineral diagenesis.

Transition Depth (K-Feldspar Rich vs Poor)

  • Transition from smectite to illite is faster in k-felspar rich sands compared to k-felspar poor sands.
  • The transition typically happens faster in mudstones compared to sandstones.
  • This is similar for shales.

Actual Transition Depths

  • Actual transition depths for illite/smectite vary among North Sea basins.

US Basin Smectite Content

  • US basin examples, like the Norwal and Peeler basins have variable geothermal gradients.
  • This difference relates to differences in smectite content as a proxy for authigenic illite.

Forties Field, North Sea

  • The Forties Field, the largest field in the North Sea, show chlorite presence at depth as kaolinite diminishes.
  • Lithic clasts for Mg and Fe could the source.

Porosity and Permeability Variations

  • Enormous variations of porosity and permeability exist in samples from marine sediments.
  • These variations can be tied to local diagenesis.
  • Trends exist for Turbidites, Siliciclastics, and Carbonates.

Sample Permeability

  • Different types of samples have different hydraulic conductivity values.
  • Data compiled for a set of samples.

Glauconite Sandstone

  • Quite separately from the formation of subsurface cements, sandstones can be impregnated by authigenic glauconite.
  • Authigenic glauconite surfaces are associated with sedimentary hiatus periods.

Glauconite Formation

  • Glauconite forms from the reaction of iron, potassium and silica in seawater during periods of very slow deposition.
  • It can form in sands, clays, impure limestones, and chalk and is "authigenic."
  • It forms from fecal pellets, peloids, and clays, but can also form directly from seawater.
  • Glauconite forms often as a granular mineral.

Glauconite Properties

  • Glauconite-rich sandstones are called greensands.
  • Greensands can weather to various rust colors.
  • Glauconite is associated with phosphoric minerals and has been used in fertilizers.
  • Two main chemical explanations for glauconite formation highlight microbial activities and chemical reactions at the sediment-water interface, with the formation of Fe-smectite and mica-rich glauconite smectite mixtures.

Weathering and Clay Formation

  • Weathering significantly alters rocks before they are transported to the basin of deposition.
  • Weathering processes derive clays from solid-rock.
  • Clays are transported from the polar to equatorial based on precipitation, evaporation rates and dominant plant life.

Time Dependency of Smectite to Illite

  • The smectite to illite transition is not just affected by temperature but also sufficient time.
  • Mixed layer clays are the initial phase of this transition, with pore water solution, dissolution causing mineral rearrangements.

Chemical Changes in Clay Transformation

  • Smectite clay transformations require diffusive chemical changes at the molecular level.
  • Transformations occur as certain chemical species (Si/Fe/Mg/Na) diffuse into and out of the illite/smectite mixtures.
  • Chemical changes occur as these species are added and removed.

Shale Diagenesis

  • Maturity effects on mineral phases include compaction, clay de-watering/re-alignment, transformation, illitization, overgrowth/cement precipitation, increase in clay crystallinity and preferred orientation, strengthened and stiffed properties, and porosity reduction and development.

Organic Matter Diagenesis

  • Maturity effects on organic matter involve removal of functional groups, catagenesis (oil generation), oil migration and cracking, aromatization, and reorganization of skeletal structure.
  • These effects impact microstructures, such as loss of fluorescence, increase in reflectance, and spectral variation.

Overmature Marcellus Shale

  • This overmature Marcellus shale, with 3-7% TOC and Ro=4+, shows weak electrical conductivity and has resistivity of <0.1 ohm-meter.

Core Mineralogy

  • Similar mineral compositions include quartz, illite, calcite, albite, pyrite, and chlorite.
  • The TOC content is also well documented in the same core.

Bitumen and Proto-Graphite

  • Overmature Marcellus shale contains primarily bitumen, with proto-graphite, connected to minute pore spaces and grain boundaries, which may be conductive.

Variations in Shale Properties

  • These variations (from the Marcellus and Canning Basins examples) in the impact of diagenesis on the macroscopic properties of organic and mineral components, influence how the timing of formation of different diagenic mineral phases impact fabric formation, strength, and anisotropy. The time of arrival of these mineral components may make a substantive difference to end results.

Summary of the Presentations

  • The slides address a broad range of topics, all related to the diagenesis of clastic sedimenets, including carbonates and shales.
  • Topics range from formation of grains and sediments in various environments, to processes forming different classes of cement in sedimentary rock.
  • A series of presentations were reviewed on how the diagenesis of important subsurface rock components (clays and organic matter) control various rock properties like porosity, permeability, and conductivity.

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